High-temperature materials processing has become increasingly important as a technique for manufacturing many types of special performance industrial components. Precision shapes comprising a large variety of metallic and non-metallic materials selected for certain desirable performance characteristics are commonly manufactured using a high-temperature environment. For example, powder metallurgy is a familiar process for making a wide range of components and shapes from a variety of metals and alloys initially in powder form. The process utilizes pressure and heat to form the powders into precision shapes that require minimal secondary finishing.
In another familiar industrial application, one or a mixture of ceramic oxides and other ceramic-like compositions can be sintered to form a composite product of desired shape. Although sintering occurs in loose powders, it is commonly enhanced by compacting the powder. In some applications, compacting is performed at room temperature, and the resulting compact is then sintered at elevated temperature without application of pressure. In other applications, a hot pressing process is used in which compacting is carried out at elevated temperature.
Typically, these and other types of high-temperature thermal processing operations are conducted in batch-style furnaces. A principal advantage of a batch-style furnace is that it has no moving internal parts. The inner walls of a batch-style furnace and any platform for supporting the product being processed can be readily fashioned from highly refractory materials which retain their structural stability even at the elevated processing temperatures of 1200.degree. C. and above. Ceramic materials and graphite, for example, are commonly employed for such high-temperature applications. These materials, however, while highly refractory, are also typically quite brittle and generally are incapable of supporting heavy loads or retaining structural stability under significant tensile forces. Such material limitations have greatly restricted the use of these materials which, in turn, has limited innovation in high-temperature furnace design.
More particularly, it is well-recognized that conventional batch-type high-temperature furnaces are highly energy inefficient with relatively low throughputs and long processing times. These batch-type furnaces require long heating and cooling cycles of typically three-six hours each, even though actual processing time for many non-oxide ceramic materials at peak temperature is generally only about thirty-ninety minutes. During operation, substantial heat loss occurs as the entire furnace and its contents must repeatedly be brought up to temperature, held there for the requisite processing time, then gradually cooled, only to be reheated during the next processing operation. As a result, it has been costly and time-consuming to carry out such operations, as well as requiring a heavy capital investment in furnace equipment, which capital costs must be amortized over a relatively low product volume.
More recently, there has been interest in continuous processing operations as a way to overcome some of the drawbacks of the batch furnace. Thus, U.S. Pat. No. 3,762,014 (Klein) describes a process for fabricating anode preforms by continuously sintering deposits of tantalum to a very thin tantalum foil strip utilizing a drive apparatus which is adjacent to but completely external of the furnace. The Klein apparatus comprises a continuous belt driven by rotating drums to carry the foil strip and deposits into a furnace maintained at a temperature of 1800.degree. C.-2500.degree. C. The belt speed is adjusted so that a given section of the foil strip remains inside the furnace sufficiently long (e.g. 1-60 minutes) for the sintering to be completed. While inside the furnace, the foil strip is supported on a stationary, horizontally-disposed, refractory support member (reference numeral 40 in FIG. 4 of the Klein patent). The continuous belt (reference numeral 31 in FIG. 4 of the Klein patent) never enters the sintering furnace (reference numeral 37).
A different approach to continuous sintering utilizes a belt furnace design in which articles to be heated, such as discrete shaped containers holding metal or ceramic powder, are placed on a continuous conveyer belt to be carried into, through and out of a furnace preheated to appropriate temperature. In general, such a belt furnace design for sintering silicon nitride is described in an article by Dale E. Wittmer et al. entitled "Continuous and Batch Sintering of Silicon Nitride" appearing the American Ceramic Society Bulletin, vol. 72, no. 6 (June 1993) at pages 129-137, and in a second article by Dale E. Wittmer et al. entitled "Comparison of Continuous Sintering to Batch Sintering of Si.sub.3 N.sub.4 " appearing in Ceramic Bulletin, vol. 70, no. 9 (1991), both of which are incorporated herein by reference.
There are several reasons that sintering in belt furnaces is more cost effective than sintering in batch furnaces. The cycle time and thermal load remain substantially constant through the belt furnace, whereas the cycle time for batch furnaces increases with increasing thermal load. To reduce thermal lag in a batch furnace, the heating rate must be reduced to minimize thermal gradients across the load cross section. Larger thermal loads also take longer to cool in batch furnaces, whereas the cooling rate for belt furnaces depends on transport speed through the furnaces.
It is also well-known and accepted that the larger the batch furnace, the lower the expected furnace yield. The reduced yield is primarily due to the large thermal lag that exists between the parts and the furnace elements and the large temperature gradient that exists in the load during heating and cooling. To minimize temperature gradients, the heating and cooling rates need to be controlled, which adds to the sintering cycle time, and may have significant effects on the microstructure, surface reaction layers, and properties of the resulting sintered parts.
Furthermore, in belt furnaces, changes in operating conditions and onset of furnace failure are more easily detected and corrected than in batch furnaces. There is less product lost and less downtime to affect total production in the case of furnace upsets in belt furnaces compared with batch furnaces. When a problem is detected in a belt furnace, the furnace can be purged of the parts in the run, cooled, repaired, and brought back to operation in less than four to eight hours. Direct and indirect product losses can be kept to a minimum for the belt furnace.
Despite the seeming advantages, however, the use of continuous belt furnaces for high-temperature applications has been limited in practice. First, any components of a belt furnace which are permanently located in, or even temporarily pass through, the inside of the heated furnace must be highly refractory and able to withstand the extremely high operating temperatures of 1200.degree. C. and above, often more than 2000.degree. C., without melting, vaporizing, decomposing or losing structural stability. Although materials such as graphite and some ceramics are sufficiently refractory for such applications, they have also typically proven too brittle to be effectively formed into a moving belt capable of transporting objects into and through the heated furnace interior.
A tantalum ribbon belt, as shown for example in the Klein patent, can be formed with sufficient high-temperature ductility to survive a single furnace pass at low load capacity, as shown in Klein. But, the tantalum belt in the Klein patent was relatively narrow (about two inches wide), and no more than about a six-inch length of the belt was inside the heated furnace interior at any one time. Also, the very thin belt in the Klein patent only had to support relatively small, low-weight frozen slugs of metallic powder. Thus, it has been determined that the load capacity of the Klein tantalum belt was no more than about one-two ounces per square inch of belt surface, and the tensile rating was six pounds or less for a two-inch width of the belt. Furthermore, the tantalum belt in the Klein patent only needed to survive a single pass through the furnace and did not have to be sufficiently flexible to conform to the rounded outer surfaces of a pair of rotating drums, as is the case with a continuous belt drive. Because the heated tantalum belt oxidizes rapidly, becomes increasingly brittle, and distorts relatively easily, such a belt would be expected to fail relatively quickly in a high-temperature continuous belt application.
Another metal sometimes used for fabricating the belt of a high-temperature belt furnace is tungsten. Tungsten in pure form has relatively good high-temperature ductility and adequate strength for belt applications. But, at furnace operating temperatures, tungsten becomes highly reactive with carbon and oxygen, forming relatively brittle compounds. As a consequence of this limitation, tungsten belt furnaces must be kept free of atmospheric oxygen, and the tungsten belt must be cooled below reactive temperature before exiting the furnace. Inside the furnace, the atmosphere must also be kept free of carbon, which prevents the use of conventional, relatively inexpensive, and relatively thermally-efficient carbon interiors together with conventional fiber insulation, and also restricts the use of a carbon atmosphere inside the furnace. Instead, furnace interiors must be made of expensive tungsten construction to prevent contamination of the tungsten belt. Moreover, because tungsten is not a thermal insulator, conventional fiber insulation cannot be placed externally adjacent to the hot tungsten interior furnace wall. Instead, multiple tungsten shells are typically used to insulate such furnaces, which is expensive and thermally inefficient.
These and other problems with and limitations of the prior art are overcome with the improved high-temperature furnace apparatus of this invention.